Design of Integrated Bragg Grating-Based Filters for Optical Communications

Project Staff: Thomas Murphy, Juan Ferrera, J. Todd Hastings, M. Jalal Khan, Michael H. Lim, Professor Hermann Haus, and Professor Henry I. Smith.

Sponsors:Air Force Office of Scientific ResearchContract F49620-96-1-0126

As described in related reports, we have developed many new lithographic techniques specifically tailored to meet the needs of integrated Bragg gratings. As a vehicle for demonstrating these techniques, we are in the the process of developing two novel devices which could play an important role in future optical networks.

The first device we are developing, depicted in Fig. 40, is based upon quarter-wave-shifted Bragg gratings. When a quarter-wave shift is introduced in an otherwise uniform Bragg grating, the resultant structure behaves as an optical resonator, similar to a Fabry-Perot cavity or a ring resonator. The structure is designed such that only one wavelength channel from a multi-wavelength system will excite the resonator. The device therefore acts as an add/drop filter, enabling the addition or extraction of a channel from the bus waveguide, while leaving all other channels unaffected. The second resonator, located below the bus, ensures that there is no appreciable reflection of the resident channel into the input port of the device.

The device depicted in Fig. 40 is a first-order filter, which has the characteristic Lorentzian bandpass response expected for a single-pole resonator. By cascading multiple resonators, it is possible to achieve more complicated higher-order filters. To address the complex design challenges of these filters, we have developed an equivalent-circuit model that maps the Bragg grating based waveguides onto equivalent electrical circuits consisting of resistors, inductors, and capacitors. Once this association has been made, the spectral response of the filter may be engineered using standard circuit tables. For example, we have used the equivalent circuit technique to design third-order Butterworth filters. Once we have mapped the electrical parameters to their corresponding optical parameters, we use computer simulations to calculate the physical dimensions of the waveguides and gratings that yield the desired values for these optical parameters. This dual approach of using analytic techniques and computer simulations to design devices enables us to generate detailed design tables which take into account, and allow for, unpredictable variations in the manufacturing sequence.

Figure 40: A schematic diagram of the resonator-based add/drop filter. Several independent data channels, each at a different wavelength, travel along the bus waveguide. One channel excites the quarter-wave-shifted Bragg resonator, and is tapped off in the upper port of the device. This device may be operated in reverse, allowing one to selectively add a channel to the bus.

The second device that we are developing, depicted in Fig. 41, is a simpler Bragg-grating filter. The gratings in this device are long, uniform structures without quarter-wave shifts. In this implementation, each of the Bragg gratings acts like a wavelength-selective reflector. The two identical Bragg gratings are integrated in a Mach-Zehnder interferometer, which separates the signals reflected from the gratings from the input signal. Light is launched in the upper left port of the device, and split equally by the coupler. A portion of the light is reflected by the identical Bragg gratings located in the arms of the interferometer. Provided the arm lengths are matched, these reflected signals recombine and emerge in the lower left port of the device.

Depending upon the characteristics of the Bragg grating, the filter can be configured to perform many different functions. For example, by appropriately selecting the length and depth of the Bragg grating, the reflection spectral response can be made to have a bandpass shape. With this configuration, the device performs as an add/drop filter: one wavelength channel is reflected by the gratings, while all other channels pass-through unaffected. The same channel may be simultaneously added by launching it in the upper right port.

Another useful device may be realized by making the Bragg grating very shallow, such that peak reflectivity is small. In this regime, the reflection spectral response of the Bragg grating is well approximated by the Fourier transform of the grating shape. Thus, for a Bragg grating of length L, the resultant spectral response has the characteristic "sinc" response, with a bandwidth inversely proportional to L. This device is an ideal pre-detection filter, since it can be engineered to have a spectral response which is matched to that of an on-off modulated data signal. Such a "matched filter" is predicted to yield the optimal signal-to-noise ratio when it is used to filter white noise from a binary data signal. Theoretical predictions indicate that such a matched filter can yield a twofold increase in the sensitivity of a communications system, when compared with commonly used (nonmatched) filters. This means that the same error-rate performance can be achieved with only half the optical power currently used.

The devices described here illustrate the rich variety of optical filters that can be constructed using integrated Bragg gratings. We are currently in the process of building these devices, using the fabrication techniques described in the prior section.

Figure 41: A schematic diagram of Bragg gratings integrated in a Mach-Zehnder interferometer. The identical gratings located in the opposite arms are designed to reflect a portion of the incident light. The filtered signal emerges in the lower left port.